Geotechnical investigation and design
A major tunnel project must start with a comprehensive investigation of ground conditions by collecting samples from boreholes and by other geophysical techniques. An informed choice can then be made of machinery and methods for excavation and ground support, which will reduce the risk of encountering unforeseen ground conditions. In planning the route, the horizontal and vertical alignments can be selected to make use of the best ground and water conditions. It is common practice to locate a tunnel deeper than otherwise would be required, in order to excavate through solid rock or other material that is easier to support during construction.
Conventional desk and preliminary site studies may yield insufficient information to assess such factors as the blocky nature of rocks, the exact location of fault zones, or the stand-up times of softer ground. This may be a particular concern in large-diameter tunnels. To give more information, a pilot tunnel (or "drift tunnel") may be driven ahead of the main excavation. This smaller tunnel is less likely to collapse catastrophically should unexpected conditions be met, and it can be incorporated into the final tunnel or used as a backup or emergency escape passage. Alternatively, horizontal boreholes may sometimes be drilled ahead of the advancing tunnel face.
Other key geotechnical factors:
- "Stand-up time" is the amount of time a newly excavated cavity can support itself without any added structures. Knowing this parameter allows the engineers to determine how far an excavation can proceed before support is needed, which in turn affects the speed, efficiency, and cost of construction. Generally, certain configurations of rock and clay will have the greatest stand-up time, while sand and fine soils will have a much lower stand-up time.
- Groundwater control is very important in tunnel construction. Water leaking into a tunnel or vertical shaft will greatly decrease stand-up time, causing the excavation to become unstable and risking collapse. The most common way to control groundwater is to install dewatering pipes into the ground and to simply pump the water out. A very effective but expensive technology is ground freezing, using pipes which are inserted into the ground surrounding the excavation, which are then cooled with special refrigerant fluids. This freezes the ground around each pipe until the whole space is surrounded with frozen soil, keeping water out until a permanent structure can be built.
- Tunnel cross-sectional shape is also very important in determining stand-up time. If a tunnel excavation is wider than it is high, it will have a harder time supporting itself, decreasing its stand-up time. A square or rectangular excavation is more difficult to make self-supporting, because of a concentration of stress at the corners.
Choice of tunnels versus bridges
For water crossings, a tunnel is generally more costly to construct than a bridge. However, navigational considerations may limit the use of high bridges or drawbridge spans intersecting with shipping channels, necessitating a tunnel.
Bridges usually require a larger footprint on each shore than tunnels. In areas with expensive real estate, such as Manhattan and urban Hong Kong, this is a strong factor in favor of a tunnel. Boston's Big Dig project replaced elevated roadways with a tunnel system to increase traffic capacity, hide traffic, reclaim land, redecorate, and reunite the city with the waterfront.
The 1934 Queensway Tunnel under the River Mersey at Liverpool was chosen over a massively high bridge for defense reasons; it was feared that aircraft could destroy a bridge in times of war. Maintenance costs of a massive bridge to allow the world's largest ships to navigate under were considered higher than for a tunnel. Similar conclusions were reached for the 1971 Kingsway Tunnel under the Mersey. In Hampton Roads, Virginia, tunnels were chosen over bridges for strategic considerations; in the event of damage, bridges would prevent US Navy vessels from leaving Naval Station Norfolk.
Water-crossing tunnels built instead of bridges include the Holland Tunnel and Lincoln Tunnel between New Jersey and Manhattan in New York City; the Queens-Midtown Tunnel between Manhattan and the borough of Queens on Long Island; the Detroit-Windsor Tunnel between Michigan and Ontario; and the Elizabeth River tunnels between Norfolk and Portsmouth, Virginia; the 1934 River Mersey road Queensway Tunnel; the Western Scheldt Tunnel, Zeeland, Netherlands; and the North Shore Connector tunnel in Pittsburgh, Pennsylvania.
Other reasons for choosing a tunnel instead of a bridge include avoiding difficulties with tides, weather, and shipping during construction (as in the 51.5-kilometre or 32.0-mile Channel Tunnel), aesthetic reasons (preserving the above-ground view, landscape, and scenery), and also for weight capacity reasons (it may be more feasible to build a tunnel than a sufficiently strong bridge).
Some water crossings are a mixture of bridges and tunnels, such as the Denmark to Sweden link and the Chesapeake Bay Bridge-Tunnel in Virginia.
There are particular hazards with tunnels, especially from vehicle fires when combustion gases can asphyxiate users, as happened at the Gotthard Road Tunnel in Switzerland in 2001. One of the worst railway disasters ever, the Balvano train disaster, was caused by a train stalling in the Armi tunnel in Italy in 1944, killing 426 passengers. Designers try to reduce these risks by installing emergency ventilation systems or isolated emergency escape tunnels parallel to the main passage.
Project planning and cost estimates
Government funds are often required for the creation of tunnels. When a tunnel is being planned or constructed, economics and politics play a large factor in the decision making process. Civil engineers usually use project management techniques for developing a major structure. Understanding the amount of time the project requires, and the amount of labor and materials needed is a crucial part of project planning. The project duration must be identified using a work breakdown structure (WBS) and critical path method (CPM). Also, the land needed for excavation and construction staging, and the proper machinery must be selected. Large infrastructure projects require millions or even billions of dollars, involving long-term financing, usually through issuance of bonds.
The costs and benefits for an infrastructure such as a tunnel must be identified. Political disputes can occur, as in 2005 when the US House of Representatives approved a $100 million federal grant to build a tunnel under New York Harbor. However, the Port Authority of New York and New Jersey was not aware of this bill and had not asked for a grant for such a project. Increased taxes to finance a large project may cause opposition.